Yersinia pestis, an etiological agent of the acute diseases bubonic and pneumonic plague and one of the most devastating epidemic-causing bacteria experienced by mankind, is now classified as a re-emerging human pathogen by the WHO. The potential for contagion, lack of an effective vaccine, and emergence of multiple antibiotic- resistant strains place Y. pestis at the top of the U.S. select agent list as a potential bioterrorism agent. Our long-term goal is to elucidate molecular mechanisms underlying the acute bacterial infectious process of Y. pestis . The more immediate objective is to identify and evaluate new and existing antigens of Y. pestis to develop a new generation plague vaccine. The complete genome sequence of Y. pestis is now known. Four aims are proposed. In Aim 1 we will prepare a Braun/murein lipoprotein (lpp)-minus mutant of Y. pestis , based on our recent data that the patented Ipp isogenic mutants of Salmonella Typhimurium and of Y pseudotuberculosis are avirulent in mice and provide protection against challenge with the wild-type bacterium. We will examine these mutants for immunological responses in mice to develop a live attenuated Y. pestis vaccine or use Y. pseudotuberculosis as a carrier for Y. pestis antigens. Aim 2 will identify differentially or exclusively expressed genes (potentially virulence-associated) of Y. pestis by genomics and proteomics to evaluate new antigens for use in a recombinant subunit plague vaccine. Aim 3 will examine the virulence potential of selected in vivo-expressed genes of Y. pestis by developing isogenic mutants and evaluating them for lethality in a mouse model and assess selected antigens' ability to provide immunity against Y. pestis challenge. Aim 4 will examine potential use of the Ipp-minus mutants of Y. pseudotuberculosis and S. Typhimurium as carriers to deliver Y. pestis antigens by expressing selected genes either from a plasmid under an inducible promoter and/or chromosome of attenuated Y. pseudotuberculosis/S. Typhimurium or by DMA vaccination. Alternatively, a vaccine strain of S. Typhi (Ty21a) could also be used. We believe these multiple approaches will identify candidate antigens for a use in a new, efficacious plague vaccine.